Micromirrors have emerged as key components for optical microelectromechanical systems (MEMS) applications, including switching and scanning operations used, for example, in optical signal routers and projection displays. Such micromirrors are electrostatically tilted using capacitive plate or vertical comb actuators which can be fabricated underneath the micromirror, thereby allowing for arrays having a high fill factor. These electrostatic actuators operate with a voltage applied between a pair of capacitor plates or interdigitated electrostatic combs to produce an electrostatic force of attraction that urges a moveable capacitor plate or electrostatic comb towards a fixed capacitor plate or comb thereby tilting the micromirror.
One limiting factor, however, for these capacitive plate and vertical comb electrostatic actuators is an electrostatic pull-in instability which occurs when the electrostatic force of attraction exceeds a restoring force of a linear torsion spring which has heretofore been used to suspend the micromirror. When pull-in occurs once the applied voltage reaches a pull-in value, the tilted micromirror can no longer maintain an equilibrium position and will abruptly move, or pull in, to its fully actuated position. This behavior can be advantageous in a digital micromirror having only two stable actuation positions (i.e. tilted and untilted). However, for micromirrors which must be continuously and controllably tilted, the pull-in instability severely limits a tilt scan range of the micromirror.
Additionally, once the micromirror has been pulled in to its fully actuated position, the voltage required to maintain that position is less than the pull-in value. As a result, the micromirror will not move from the fully actuated position until the applied voltage has been reduced below the pull-in value. The result is that the tilting movement of the micromirror exhibits an undesirable hysteretic behavior.
The present invention overcomes the limitations of the prior art by providing a progressive linkage which acts as a non-linear spring to substantially alleviate or eliminate the electrostatic pull-in instability and hysteretic behavior of the micromirror, thereby increasing a range of tilt of the micromirror.
These and other advantages of the present invention will become evident to those skilled in the art.